This invention relates to a method and an apparatus for slitwise exposure of a photosensitive member in copying machines.
An exposure system is known in which an original to be copied is placed stationarily in planar form and a slitwise exposure of a photosensitive member with a light image of the original takes place at a given location by moving the surface of the photosensitive member in a given direction at a uniform rate.
The present inventor has previously proposed a method of slitwise exposure for a photosensitive member in which the surface of a focusing lens which is located on the object side is disposed in direct opposing relationship with a glass pane on which an original is placed, while a pair of reflecting mirrors are disposed on the image side of the focusing lens and are moved to effect a slitwise exposure of the photosensitive member (Japanese Laid-Open Patent Application No. 102,041/1978).
The present invention relates to an improvement of the method disclosed in this previous application. Reference is now made to FIGS. 1 and 2 to provide a brief summary of the disclosed method and related problems which are to be overcome by the present invention.
Referring to FIG. 1, there is shown a glass pane 1 on which an original is placed in planar form, with its surface carrying figures and characters facing down. The glass pane is stationary in a copying machine. A focusing lens 2 is disposed below the central region of the glass pane 1 so that its optical axis is perpendicular to the plane of the glass pane. The focusing lens is held stationary in the machine, and remains stationary except for its displacement which is required in changing the magnification when the copying machine is of a variable magnification type.
A pair of reflecting mirrors A, B are disposed on the image side of the focusing lens 2, and each comprises a plane mirror of a rectangular form with its length extending in a direction perpendicular to the plane of the sheet of drawing. These mirrors have specular surfaces AM, BM which are disposed at given angles with respect to the plane of the glass pane 1 and at a given angle relative to each other so that the specular surfaces are disposed opposite to each other. When light from the original placed on the glass pane 1 impinges on the focusing lens 2, part of the imaging flux from the lens is successively reflected by the specular surfaces AM and BM onto the photosensitive member 3. The positions of the reflecting mirrors A, B are chosen such that the part of imaging flux is focused slitwise on the peripheral surface of the photosensitive member at a slitwise exposure station S.sub.L which represents a given location in the space of the machine. The slitwise exposure station S.sub.L is defined by a slit member, not shown.
The design of the slitwise exposure station S.sub.L determines the width S.sub.S of a slit-shaped portion of the original which is being scanned. Specifically, when the plane mirrors A, B are in their start positions shown in solid lines, the images of a slit width S.sub.S of the original at the leftmost portion of the glass pane 1 are projected onto the slitwise exposure station S.sub.L.
The reflecting mirrors A, B are driven integrally to move in a direction indicated by an arrow C at a uniform rate. In a corresponding manner, successive portions of the original are focused onto the slitwise exposure station S.sub.L of the photosensitive member 3, and when the reflecting mirrors A, B reach their stop positions shown in phantom lines, the rightmost portion of the original corresponding to the slit width S.sub.S is projected onto the slitwise exposure station S.sub.L. In other words, as the reflecting mirrors A, B move in an integral manner, the image of the original moves across the slitwise exposure station S.sub.L. Hence, by rotating the photosensitive member 3 in a direction indicated by an arrow and at a rate which corresponds to the speed of movement of the image, the photosensitive member 3 is slitwise exposed to form a latent image thereon which corresponds to the original.
In FIG. 1, point K.sub.O at the left-hand end of the glass pane 1 represents the starting point of the scanning of the original, and corresponds to a point K.sub.1 where the exposure is started in the slitwise exposure station S.sub.L.
The original scanning slit moves in a direction indicated by an arrow D through a distance of S.sub.S +S.sub.O where S.sub.O represents the effective length of the glass pane 1. The movement of the original scanning slit through such distance scans a region of the original having a length S.sub.O +2S.sub.1 which will be hereinafter referred to as an effective copying region. During the time the slitwise scanning of the original takes place, the effective copying region may be entirely illuminated with a given illumination distribution or may be slitwise illuminated by an associated illumination system in timed relationship with the slitwise scanning of the original.
The distribution of illumination is chosen to satisfy the requirement that a portion of the photosensitive member 3 which defines the exposure station is entirely exposed under the same optical conditions. The requirement remains unchanged if the slitwise illumination is utilized. The lengthwise distribution of illumination which is obtained from a slit-shaped illuminating unit may be made to be a desired one by regulating the light emitting intensity of an illuminating lamp, or by controlling the amount of light emitted by an illuminating light in accordance with the movement of the slitwise illuminating assembly.
The described slitwise exposure technique is applicable to a copying machine of a variable magnification type, since the displacement of the reflecting mirrors A, B which is required in changing the magnification is in one dimension to enable such displacement to be achieved with a relatively simple mechanism, and since the speed of movement of reflecting mirrors which is required to effect the slitwise exposure of the photosensitive member remains unchanged regardless of the magnification.
The imaging flux which exposes the photosensitive member 3 changes its position on the reflecting mirrors A, B in a direction perpendicular to the length of the mirrors and parallel to the specular surfaces, as the reflecting mirrors A, B move. This explains why the reflecting mirrors A, B require a width as indicated in FIG. 1 even though the width of the imaging flux which participates in the slitwise exposure has a reduced width. As will be apparent from FIG. 1, the reflecting mirror B has an increased width for its specular surface, though the corresponding width is not as great for the reflecting mirror A. The described slitwise exposure technique suffers from the problem that the reflecting mirror B having such an increased size must be moved adjacent the photosensitive member 3. Another problem results when the exposure technique is applied to a copying machine of a variable magnification type since in this instance the reflecting mirror A as well as the reflecting mirror B must have an increased width for its specular surface. This will be considered in more detail with reference to FIG. 2.
In FIG. 2, the focusing lens 2 and the reflecting mirrors A, B in their start positions are shown in chain lines when the magnification is unity. They are shown in solid lines and broken lines corresponding to the start and the stop positions when a magnification other than unity is utilized. The position of the focusing lens 2 which it assumes when the magnification is different from unity is shown in solid line. It is to be noted that the orientation of the focusing lens 2 and the reflecting mirrors A, B for a magnification other than unity remain the same as shown in FIG. 1. As will be apparent from comparison with FIG. 1, the reflecting mirror A as well as the reflecting mirror B needs an increased width for its specular surface.
To facilitate the understanding of the invention which will be described later, the displacement of the focusing lens system 2 and the reflecting mirrors A, B as magnification is changed will be considered.
To this end, x- and y-axis will be chosen as shown. Specifically, the x-axis lies in the plane of the drawing and extends parallel to the plane of the glass pane 1 while the y-axis is parallel to the optical axis of the focusing lens 2. A direction which is perpendicular to the plane of the sheet of the drawing will be designated as the z-axis. A displacement of the focusing lens 2 which occurs as a magnification is changed will be represented by x.sub.h, y.sub.h and z.sub.h which represents the x-, y- and z-axis component thereof. In a similar manner, a displacement of the reflecting mirrors A, B will be represented by their x- and y-axis components x.sub.M, y.sub.M. It is to be noted that the component z.sub.h is not shown in FIG. 2. The displacement of the mirrors A, B is represented by .delta..sub.O. Obviously, .delta..sub.O =.sqroot.x.sub.M.sup.2 +y.sub.M.sup.2.
A displacement of the focusing lens 2 in the direction of the x-axis is required to maintain the same positional relationship between the original scan start point K.sub.O and the exposure start point K.sub.1 as between when the magnification is unity and when the magnification is not unity. A displacement of the focusing lens 2 in the direction of the z-axis is necessary in order to maintain the marginal edge of an electrostatic latent image formed on the photosensitive member 3, as viewed in the direction of the z-axis, when the magnification is not unity in alignment with that formed when the magnification is unity.
A simple geometooptical calculation yields: ##EQU1## where m represents a magnification, that is, the ratio of the size of a copy to the size of the original, x.sub.k the distance between the scan start point K.sub.O and the optical axis, .theta..sub.O an angle which a ray of light passing through the optical axis of the focusing lens 2 forms with the y-axis at its point of impingement on the photosensitive member, f the focal length of the lens 2, and z.sub.k one-half the effective width of the glass pane 1 in the direction of the z-axis. It will be seen that the angle .theta..sub.O /2 represents an angle formed between the direction C of movement of the reflecting mirrors A, B and the x-axis, and is also equal to the angle between the direction of displacement of the reflecting mirrors A, B which is necessary to effect a change in the magnification and the y-axis.
The displacement of the reflecting mirrors A, B is given by the following expressions: EQU S.sub.M =(S.sub.O +S.sub.S)/2 cos (.theta..sub.O /2), EQU S.sub.MO =m(S.sub.O +S.sub.S)/2 cos (.theta..sub.O /2)
where S.sub.M represents the displacement for a magnification of unity and S.sub.MO the displacement for a magnification other than unity.
Assuming that the reflecting mirrors A, B move with a speed V.sub.M which remains unchanged when the magnification is varied and that the peripheral surface of the photosensitive member 3 moves with a speed V.sub.O, we have EQU V.sub.M =V.sub.O /2 cos (.theta..sub.O /2)
Another problem inherent in the conventional arrangements is the fact that the direction in which the reflecting mirrors A, B move is fixed, resulting in a reduced freedom in the design of the apparatus.